1. Field of the Invention
The present invention relates generally to the art of optical inspection of specimens, such as semiconductor wafers and hard disk surfaces, and more specifically to a system for determining surface topographies in the nanometer range using optical techniques.
2. Description of the Related Art
Optical inspection techniques for specimens, such as semiconductor wafers, have assessed the relative flatness of specimen surfaces using various techniques. Surface flatness is a critical parameter used to determine the overall quality of a semiconductor wafer, and wafers having large irregular areas or small areas with radical height differences are undesirable.
Current tools available to measure wafer surface flatness include the “Magic Mirror” tool by Hologenix. The “Magic Mirror” operates by directing collimated light toward the wafer surface, wherein the collimated light source is angularly displaced from the wafer surface. The “Magic Mirror” system subsequently receives the reflected light. Light may be scattered toward or away from the detector. The “Magic Mirror” thereupon produces a two dimensional depiction of the surface of the observed semiconductor wafer, with associated light and/or dark areas depending on the type of defect. As can be appreciated, the “Magic Mirror” is a very subjective method of detecting surface contours. With different types of defects producing different optical effects, one cannot say for certain what type or size of defect is responsible for the bright or dark spot or area in the “Magic Mirror” depiction. Hence algorithms cannot conclusively provide areas of concern or threshold exceedance with reasonable degrees of certainty. The final two dimensional representation obtained from the “Magic Mirror” must be studied by an operator, and results depend on many uncontrollable factors.
An alternate method for measuring surface contours utilizes a profiler, much like a stylus on a record player, which directly contacts the semiconductor wafer surface. Such a system moves the semiconductor wafer and sensor relative to each other causing the sensor to linearly translate across the surface, thereby providing contact between the profiler and the entire surface. Movement of the profiler is recorded, and surface irregularities are detected when the profiler deflects beyond a threshold distance. The problems inherent in a profiler are at least twofold: first, a mechanical profiler contacting the wafer surface may itself produce surface irregularities beyond those present prior to the testing, and second, the time required to make accurate assessments of surface irregularities is extensive. For example, a full map of a single 200 mm wafer using a profiler may take between four and twelve hours.
A system is needed which diminishes the time required to perform surface scanning for contour differences and does not have the drawbacks inherent in previously known systems. In particular, it would be desirable to have a system for determining contours in the nanometer range which would not risk damage to the specimen surface and would be quantitative in nature, thereby allowing for computational determination of surface irregularities using threshholding without ad hoc human review.
It is therefore an object of the current invention to provide a system for determining the contours of the surface of a specimen, such as a semiconductor wafer, in the nanometer range which can perform surface irregularity determination in less time than systems previously known.
It is a further object of the current invention to provide a system for determining the contours of the surface of the wafer which does not include non-quantitative measurement techniques.
It is a further object of the current invention to provide a system for determining the contours of a wafer surface which does not increase the risk of damaging the wafer surface.